Dental cements are used to attach prosthetic devices such as inlays, onlays, crowns, or posts to tooth structures so that a diseased tooth can restore its function and aesthetics. Dental cements can also be used to attach orthodontic devices, such as orthodontic brackets or an orthodontic band, to a tooth structure to correct misalignment of teeth or adjust spaces between teeth. Dental cements can also be used as root canal sealants.
When cementing prosthetic and orthodontic devices to tooth structures, there are a variety of classes of dental cements available to dental practitioners to choose from and they include, for example, (1) a zinc phosphate cement, (2) a zinc carboxylate cement, (3) a zinc oxide eugenol (ZOE) cement, (4) a glass-ionomer cement, (5) a resin cement, and (6) a resin modified glass-ionomer (RMGI) cement. Each type of cement has its own concomitant advantages and shortcomings, as discussed next.
For example, the zinc phosphate cement utilizes the acid-base setting reaction between zinc oxide and phosphoric acid. Generally, the zinc phosphate cement is provided as a powder/liquid system, which requires manual mixing of the powder and liquid components prior to application. Manual mixing can be a messy process and often lacks consistency due to improper weighing/rationing of powder and liquid components. In addition, zinc phosphate cements have poor adhesive strength to tooth structure, high optical opacity (poor aesthetics), and have a high solubility in the oral environment. Zinc phosphate cements also possess a substantially low initial pH, which can cause irritation or sensitivity during early stages of setting.
The zinc carboxylate cement utilizes the acid-base setting reaction between zinc oxide and polycarboxylic acid. Comparatively, zinc carboxylate cements are gentler to pulp tissue and have improved adhesive property toward tooth structure, as compared to their zinc phosphate counterparts. However, zinc carboxylate cements have a short working time, high optical opacity, low mechanical strength, and require an additional conditioning step prior to cementation.
The zinc oxide eugenol (ZOE) cement utilizes the acid-base setting reaction between zinc oxide and eugenol in the presence of water to form a zinc eugenolate chelate. A ZOE cement has low mechanical strength, poor adhesive properties, poor aesthetics, and high solubility in water, thereby making it suitable for use only as a provisional or temporary cement.
The resin cement utilizes the free-radical polymerization setting reaction of methacrylate monomers. Resin cements are generally reinforced with inorganic glass fillers and possess very good mechanical strength, favorable aesthetic properties, and low solubility in water. Resin cements demonstrate excellent bond strengths to tooth structure, when used in combination with a bonding agent. As a result, resin cements are best suited for cementing aesthetic, yet brittle ceramic restorations. Nevertheless, resin cements are rather hydrophobic and sensitive to water/saliva contamination. Resin cements are also quite technique sensitive and have the potential to cause post-operative sensitivity due to their complicated bonding protocol requiring a separate bonding agent.
The glass-ionomer cement utilizes the acid-base setting reaction between a polycarboxylic acid and a fluoroaluminosilicate glass filler. In comparison to the zinc phosphate cements, the glass-ionomer cements generally demonstrate improved adhesive property, reduced solubility in water, improved mechanical strength, and have the added benefit of cariogenic properties due to long-term sustained release of fluoride. However, the setting reaction is quite sensitive to water/moisture contamination as well as desiccation during the early stage of setting. When compared to resin cements, the glass-ionomer cements are quite brittle, and their bond strength to tooth structure is substantially lower.
Various efforts have been made to combine resin cement chemistry with glass-ionomer cement chemistry to form resin modified glass-ionomer (RMGI) cements. Akahane et al. (U.S. Pat. No. 5,063,257) incorporated polymerizable monomers, a photoinitiator, a surfactant, and a reducing agent into a glass-ionomer composition to make the resulting RMGI composition curable through both an acid-base reaction and a free-radical photo-polymerization. Mitra et al. (U.S. Pat. Nos. 5,130,347 and 5,925,715) incorporated a photoinitiator system into a glass-ionomer composition and added polymerizable groups to the polycarboxylic acid through an amide linkage, to make the resulting RMGI composition curable through both an acid-base reaction and a free radical photo-polymerization. Jandourek (EP 0,329,268A2) incorporated a photoinitiator system, polymerizable monomers, and a polymerizable polycarboxylic acid into a glass-ionomer composition to make the resulting RMGI composition curable through both an acid-base reaction and a free radical photo-polymerization. Mitra et al. (U.S. Pat. No. 5,154,762) incorporated a redox initiator system, along with polymerizable monomers and polymerizable polycarboxylic acid, into a glass-ionomer composition to make the resulting RMGI composition curable through an acid-base reaction and a free radical polymerization, either by a photoinitiator or a redox initiator system. Nakaseko (U.S. Pat. No. 6,214,101) made a paste/paste RMGI composition by incorporating polymerizable monomers and encapsulated polymerization initiators into a glass-ionomer composition.
Accordingly, RMGI cement compositions combine the setting chemistries from both glass-ionomer cements and resin cements. RMGI cement compositions retain the benefit of sustained long-term fluoride release, and provide improved mechanical strength, fracture toughness, aesthetic properties, and adhesive properties over those of traditional glass-ionomer cements alone. And due to the hydrophilic and self-adhesive nature, RMGI cement compositions are generally less sensitive to moisture/saliva contamination and are less technique sensitive, as compared to resin cements. However, the bond strengths to tooth structure of prior RMGI cement compositions are still significantly lower than a resin cement used in combination with a bonding agent. Accordingly, there is a need to further improve the adhesive properties of RMGI cement compositions.